1. Field of the Invention
The present invention relates to a laser apparatus using a semiconductor laser element.
2. Description of the Related Art
Nakamura et al., xe2x80x9cInGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode,xe2x80x9d Japanese Journal of Applied Physics Part 2 Letters, vol. 37, 1998, pp. L1020 discloses a short-wavelength semiconductor laser device which emits laser light in the 410 nm band. This semiconductor laser device is constructed as follows. First, a GaN substrate is produced by forming a GaN layer on a sapphire substrate, forming a GaN layer by selective growth using a SiO2 mask, and removing the sapphire substrate. Next, an n-type GaN buffer layer, an n-type InGaN crack prevention layer, an AlGaN/n-GaN modulation doped superlattice cladding layer, an n-type GaN optical waveguide layer, an undoped InGaN/n-InGaN multiple-quantum-well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, an AlGaN/p-GaN modulation doped superlattice cladding layer, and a p-type GaN contact layer are formed on the GaN substrate.
However, the output power of the above semiconductor laser device in the fundamental transverse mode is at most about 30 mW. In addition, current injection type semiconductor laser devices formed as above deteriorate with elapse of time, due to diffusion of dopants such as magnesium and anticipated short-circuit currents. Therefore, it is difficult to increase lifetimes of the current injection type semiconductor laser devices. In particular, when the indium content in the InGaN active layer is increased in order to obtain laser light of a longer wavelength than the green wavelength, the characteristics of the crystal deteriorate, and therefore the lifetime decreases. That is, it is difficult to obtain high output power from the semiconductor laser devices having an indium-rich InGaN active layer.
On the other hand, in the conventional semiconductor-laser excited solid-state laser apparatuses, it is difficult to achieve high speed modulation of laser light by directly modulating semiconductor laser elements which are provided as excitation light sources since the lifetimes of fluorescence emitted from rare earth elements constituting solid-state laser crystals are very long.
In order to solve the above-mentioned problems, U.S. Pat. Nos. 5,461,637 and 5,627,853 propose laser apparatuses in which surface-emitting semiconductor elements are excited with light. However, since these laser apparatuses utilize the thermal lens effect, i.e., the effect of increasing refractive indexes with temperature, the temperature must be raised. In addition, the above laser apparatuses are sensitive to temperature distribution, and the spatial oscillation mode is unstable. The spatial oscillation mode becomes further unstable when output power is high, since a cavity is generated in a carrier distribution due to generation of laser light with high output power (i.e., so-called spatial hole burning occurs), and refractive indexes decrease with increase in the number of carriers due to a so-called plasma effect.
Furthermore, CLEO ""99 (Conference on Lasers and Electro-Optics, 1999), post-deadline paper CPD15-1 reports a laser apparatus which emits laser light at the wavelength of 399 nm by exciting an InGaN surface-emitting semiconductor element with a N2 laser as an excitation light source at room temperature. However, this laser apparatus oscillates in a pulse mode having a frequency of 3 Hz, and continuous wave (CW) oscillation is not realized. In addition, since the N2 laser is used, the size and cost of the laser apparatus are great.
As described above, it is very difficult to realize high-output-power oscillation in a fundamental mode in the conventional laser apparatuses which use a semiconductor laser element, and to emit laser light in the wavelength range from ultraviolet to green.
An object of the present invention is to provide a reliable laser apparatus which oscillates in a fundamental mode with high output power.
According to the first aspect of the present invention, there is provided a laser apparatus comprising: a semiconductor laser element having a am first active layer made of a GaN-based compound, and emitting first laser light; and a surface-emitting semiconductor element having a second active layer made of a GaN-based compound, being excited with the first laser light, and emitting second laser light.
The above surface-emitting semiconductor element may comprise a layered structure formed of a plurality of semiconductor layers made of a plurality of GaN-based compounds, and a pair of mirrors may be arranged on both sides of the layered structure in the direction of the elevation of the semiconductor layers.
According to the second aspect of the present invention, there is provided a laser apparatus comprising: a semiconductor laser element having a first active layer made of a GaN-based compound, and emitting first laser light; a surface-emitting semiconductor element being excited with the first laser light, emitting second laser light, and having a second active layer made of a GaN-based compound and a first mirror arranged on one side of the second active layer; and a second mirror arranged outside the surface-emitting semiconductor element so that the first and second mirrors form a resonator.
The laser apparatuses according to the first and second aspects of the present invention have the following advantages.
(a) Since, according to the present invention, laser light is generated by a GaN-based compound surface-emitting semiconductor element which is excited with excitation laser light emitted by another GaNbased compound semiconductor laser element, the semiconductor laser element which emits the excitation laser light can be a broad area type semiconductor laser element, which can emit laser light having high output power (e.g., 1 to 10 watts). Therefore, laser light of hundreds of milliwatts to several watts can be obtained from the laser apparatus according to the present invention. That is, the laser apparatus according to the present invention can emit laser light with high output power in a fundamental transverse mode.
(b) Since the thermal conductivities of the GaN-based compound semiconductor elements are very great (i.e., about 130 W/mxc2x7K), compared with the thermal conductivities of the GaAs-based compound semiconductors, which are about 45.8 W/mxc2x7K, the aforementioned thermal lens effect is not caused in the GaN-based compound semiconductor elements. In addition, when an external mirror is provided; i.e., the aforementioned second mirror is provided outside the surface-emitting semiconductor element, laser oscillation can be achieved without using the thermal lens effect. Therefore, the oscillation mode is stable.
(c) Since a semiconductor laser element is used as an excitation light source, it is possible to realize a laser apparatus which is highly efficient, inexpensive, and capable of achieving the continuous wave (CW) oscillation.
(d) Since the surface-emitting semiconductor element can be directly modulated, it is possible to achieve high-speed modulation of laser light in the wavelength range from ultraviolet to green.
(e) The surface-emitting semiconductor elements used in the laser apparatuses according to the first and second aspects of the present invention are excited with light, and are therefore different from the usual semiconductor laser elements driven by current injection, in that the light-excited surface-emitting semiconductor elements are free from the aforementioned problem of the deterioration with elapse of time due to short-circuit currents caused by diffusion of dopants such as magnesium. Thus, the lifetimes of the laser apparatuses according to the first and second aspects of the present invention are long.
Preferably, the laser apparatuses according to the first and second aspects of the present invention may also have one or any possible combination of the following additional features (i) to (v).
(i) The first active layer may be made of an InGaN or GaN material, and the second active layer may be made of an InGaN material.
(ii) The first active layer may be made of an InGaN or GaN material, and the second active layer may be made of a GaNAs or InGaNAs material.
(iii) The laser apparatus according to the first or second aspect of the present invention may further comprise at least one third semiconductor laser element each of which has a third active layer made of a GaN-based compound, and emits third laser light, and the surface-emitting semiconductor element may be excited with the third laser light together with the first laser light.
(iv) The laser apparatus according to the first or second aspect of the present invention may further comprise at least one third semiconductor laser element each of which has a third active layer made of a GaN-based compound, and emits third laser light, and the surface-emitting semiconductor element may be excited with fourth laser light which is produced by polarization coupling of the first and third laser light.
(v) The second active layer may include a plurality of quantum wells. In particular, it is preferable that the number of quantum wells included in the second active layer is twenty or more.